1. Field of the Invention
The present invention relates to a filter using piezoelectric resonators, and in particular to a filter using piezoelectric resonators to be used in wireless circuits of mobile communication terminals such as mobile phones and wireless LANs.
2. Description of the Background Art
As to the components to be incorporated in mobile phones, wireless LANs, and the like, there is a need for a further reduction in size and weight and improved performance. As one of the components which meet such a need, there exists a filter using piezoelectric resonators composed of a piezoelectric material (see Japanese Patent No. 2800905). An example of the filter will be described below with reference to drawings.
Before describing the configuration of the filter, first, the structure of a piezoelectric resonator used in the filter will be described. FIG. 21 is a perspective view showing the basic structure of a piezoelectric resonator. FIG. 22 is a cross-sectional view taken along line B—B of FIG. 21. FIG. 23A is a diagram showing a circuit symbol of the piezoelectric resonator and FIG. 23B is an equivalent circuit diagram of the piezoelectric resonator. FIG. 24 is a diagram showing the frequency characteristics of the piezoelectric resonator.
As shown in FIG. 21, a piezoelectric resonator 210 is constructed such that an upper electrode layer 211, a piezoelectric material layer 212, a lower electrode layer 213, an insulating material layer 214, and a cavity 215 are formed on/in a substrate 216 such as silicon or glass. The cavity 215 is provided in the substrate 216 so as not to penetrate through the substrate 216. The insulating material layer 214 made of silicon dioxide (SiO2), silicon nitride (Si3N4), or the like is formed on the substrate 216 so as to cover the cavity 215. On the insulating material layer 214, there is formed the lower electrode layer 213 made of molybdenum (MO), aluminum (Al), silver (Ag), tungsten (W), platinum (Pt), or the like. On the lower electrode layer 213, there is formed the piezoelectric material layer 212 made of aluminum nitride (AlN), zinc oxide (ZnO), lithium niobate (LiNbO3), lithium tantalite (LiTaO3), potassium niobate (KNbO3), or the like. On the piezoelectric material layer 212, there is formed the upper electrode layer 211 made of the same material as the lower electrode layer 213. The upper electrode layer 211, the piezoelectric material layer 212, the lower electrode layer 213, and the insulating material layer 214 make up a vibrating portion 218. As such, the vibrating portion 218 is supported by a supporting portion 221 of the substrate 216, as shown in FIG. 22. The cavity 215 is provided to trap therein a mechanical vibration generated by the piezoelectric material layer 212.
By applying a voltage between the upper electrode layer 211 and the lower electrode layer 213, an electric field is generated in the piezoelectric material layer 212, a distortion caused by the electric field excites a mechanical vibration, and by the mechanical vibration, electrical characteristics having a resonance point and an anti-resonance point are obtained. The resonance frequency fr and anti-resonance frequency fa of the piezoelectric resonator 210 can be determined by the thickness and size of the layers of the vibrating portion 218 and parameters such as a constant of each material. The equivalent circuit of the piezoelectric resonator 210 shown in FIG. 23A is represented by a capacitance C0, an equivalent constant C1, an equivalent constant L1, as shown in FIG. 23B. Note that a resistance component is very small and thus is omitted in the equivalent circuit of FIG. 23B. Assuming that the capacitance C0 is present between the upper electrode layer 211 and the lower electrode layer 213, the capacitance C0 can be determined by the following equation (1):C0=ε0εr×S/d  (1)where ε0 is the permittivity of vacuum, εr is the relative permittivity of the piezoelectric material, S is the electrode area, and d is the thickness of the piezoelectric material.
In the equivalent circuit of FIG. 23B, based on a series resonance where impedance becomes zero and a parallel resonance where impedance becomes infinite, the resonance frequency fr and the anti-resonance frequency fa can be determined by the following equations (2) and (3), respectively:fr=1/(2π×√{right arrow over ( )}(C1L1))  (2), andfa=fr×√{right arrow over ( )}(1+C1/C0)  (3).The frequency characteristics of the piezoelectric resonator 210 are shown in FIG. 24.
FIG. 25 is a diagram showing an exemplary circuit diagram of a conventional filter using three piezoelectric resonators having the above-described frequency characteristics. In FIG. 25, a conventional filter 250 includes a series piezoelectric resonator 251, parallel piezoelectric resonators 252a and 252b, and inductors 253a and 253b. The series piezoelectric resonator 251 is connected in series between an input terminal 255a and an output terminal 255b. A first electrode of the parallel piezoelectric resonator 252a is connected to a connection point between the input terminal 255a and the series piezoelectric resonator 251, and a second electrode of the parallel piezoelectric resonator 252a is connected to a first terminal of the inductor 253a. A first electrode of the parallel piezoelectric resonator 252b is connected to a connection point between the series piezoelectric resonator 251 and the output terminal 255b, and a second electrode of the parallel piezoelectric resonator 252b is connected to a first terminal of the inductor 253b. Second terminals of the inductors 253a and 253b are grounded.
The conventional filter 250 having the above-described configuration has two types of frequency characteristics, i.e., the frequency characteristics of the parallel piezoelectric resonators 252a and 252b (solid line in graph (a) of FIG. 26) and the frequency characteristics of the series piezoelectric resonator 251 (dashed line in graph (a) of FIG. 26). Therefore, by setting each parameter of the filter 250 such that an anti-resonance point 262 of the parallel piezoelectric resonators 252a and 252b substantially corresponds with a resonance point 263 of the series piezoelectric resonator 251, a filter having a passband 265, as shown in graph (b) of FIG. 26, can be realized.
The above-described conventional filter, however, would not be able to obtain a large amount of attenuation in frequency bands before and after the passband, due to the configuration of the filter. Thus, this filter cannot be used in apparatuses, typically mobile communication terminals such as mobile phones and wireless LANs, in which one of two frequency bands needs to be passed while the other frequency band needs to be attenuated. For example, a reception filter for W-CDMA has a reception band of 2.11 to 2.17 GHz and a transmission band of 1.92 to 1.98 GHz.
In addition, in the above-described conventional filter configuration, it is necessary to provide inductors having high values between the parallel piezoelectric resonators and the ground, which necessitates an additional external circuit such as a substrate, causing an increase in the size of the filter.